Monoblock settings on a VH500

[AdrianBaker]

Hi all

My W/NG framed 500 trials had a monoblock carb on when I bought it in a non-running state. It is a 389/30 carb with the following settings:

Pilot jet: 30
Needle jet: 106
Main jet: 250
Slide Number: 3

Although the bike starts and runs well, it seems really rich low down. The plug has lovely sand coloured electrodes, but with black soot all round the outside edge - which again suggests richness. I dropped the needle down a position (from the centre) and that helped, but there is still a bit of coughing and popping on occasion. The float was sticking, so I put a new 'Stay-up' float and cut off needle in, but still it seems a bit rich. I'm not used to 500cc singles of this era though, so am I expecting too much?

I'm happy to spend time trying to get the carb set up correctly, but wondered if the above settings were about right? The main jet seems bigger than I'd expect, but higher speed running isn't the problem.

Any thoughts or ideas welcomed.
Thanks
Adrian

[dave.owen]

Go to the drags site or burlen site for correct settings
Dave

[will_curry]

According my book of words 389/030 is 1"3/16 which is perhaps a little large for a trials Ariel but
about ideal for a high compression lumpy-cammed scrambler. I use an 1"1/16 monobloc on my
trials Ariel with standard road jets: 200 main, 30 pilot, 3 1/2 slide and 106 needle jet. First kick
starting, sometimes a little black smoke coming off tickover under no load but fine when working.

[AdrianBaker]

Thanks Will. The carb choke size matches the inlet tract diameter perfectly though, which is surprising if a smaller carb is 'correct'. I do know from my racing past though that big carbs don't always make for faster lap times.
Thanks for the feedback guys - I'll have a fiddle today. I think a bigger cutaway on the slide might help.

[will_curry]

Of course it's worth trying to get the carb you've got to work properly. I'd also forgotten that
yours has an iron head which are - I think - 1"3/16 anyway. Another point which may have some
bearing is that according to my notes a 389/30 was fitted to a Greeves scrambler and so may
be rather special in the needle jet arrangement.

I've used carbs smaller than the cylinder head inlet with some success. While it can limit the
maximum flow it has the advantage of speeding up the airflow at low revs and this seems to
make the slow-speed running more reliable.

[AdrianBaker]

OK, plenty of fiddling today. New B6HS plug fitted. Starting & running OK, but too rich still.
I "converted" the slide from a 3 to nearly a 3.5 by rubbing on wet & dry on a flat surface. (Stopped just short by a few thou though to do it in stages). Dropped the needle down - clip in 2nd slot down from the top. Still too rich.
Checked the float height next (not easy on a monobloc) and it was too high by about 1/4". Managed to get this about right.
Went for a 30 mile spin. Felt good, and ran cleaner, but look at the plug afterwards

[AdrianBaker]

Next stage will be to reduce the main jet from 250 to 200 (not much effect low down though) and take a bit more off of the slide.

[AdrianBaker]

The needle has no markings on it other than a 'D' at the top above the top groove. Is this the correct needle I wonder? More research to do...

[simon.holyfield]

The needle has no markings on it other than a 'D' at the top above the top groove. Is this the correct needle I wonder? More research to do...

Do you know that the needle and needle jet are not worn?

[AdrianBaker]

quote="simon.holyfield"]
Do you know that the needle and needle jet are not worn?[/quote]

Nope, no idea and don't know how to check. It was improved by increasing the slide cutaway, so I'm getting there. If fitting the correct main jet doesn't help, I'll then replace all the jets and slide.
Never had so much trouble getting the mixture right on a bike before!